Modern electronic devices utilize integrated circuits, commonly referred to as “chips,” which incorporate numerous electronic elements. These chips are typically manufactured in a wafer format, in which numerous similar devices, known as “die” are constructed on a base made from a silicon ingot. In the past, small diameter wafers were common. Today, however, larger diameter wafers, including wafers having an eight-inch diameter (200 mm) and wafers having a twelve-inch diameter (300 mm) are typical. In addition to the increase in the diameter of the wafers, there has been a dramatic increase in the density of the circuitry on the wafers. As such, modern wafers may contain hundreds if not thousands of die making each wafer highly valuable.
It has been found, however, that the high density circuitry on these larger wafers has an increased susceptibility to stress, vibration and abrasion. In addition, it has been found, that the larger diameter wafers have an increased susceptibility to cracking and breaking. Thus, conventional handling procedures during processing, testing, storage and transportation of wafers have resulted in damage to individual die as well as to entire wafers.
As an example, during wafer level testing using a prober test machine, the wafer must be handled numerous times. These handling steps include loading the wafer onto a tray of the prober test machine using a transfer arm, picking up the wafer with a vacuum pick up device from the tray and placing it into a cassette slot and loading the cassette onto an autoloader which transfers the wafer to a stage using a transfer arm. Once on the stage, the die on the wafer are tested using the prober that must physically contact each die. After testing the transfer arm removes the wafer from the stage and returns it to the cassette. This procedure is repeated for each of the wafers to be tested.
As can be seen, the wafer must be moved numerous times during this testing process by transfer equipment such as transfer arms and vacuum pick up devices. During such movements, the wafer is subjected to numerous potential hazards. For example, if a disruption occurs during a vacuum transfer of a wafer, the wafer may be dropped resulting in cracking or breaking of the wafer. Alternatively, if the wafer is not properly positioned in the cassette, the wafer may fall out. Similarly, if the wafer is not properly aligned within the prober test machine, the prober may damage one or more die. In addition to damaging the wafer, if a wafer is dropped, it can damage processing equipment causing machine down time and maintenance costs. For example, if a wafer breaks, it leaves behind fine silicon particles which must be removed from the processing equipment to avoid causing microscratches on other wafers.
In a like manner, each wafer must be handled numerous times during other steps of wafer processing. In fact, some of the wafer processing steps may occur at different facilities which require placing the wafers in containers and shipping the wafers to another location. In this case, the wafers are typically picked up by a vacuum pick up device and lowered into a container. Again, this process raises the possibility of dropping and breaking the wafer. If the wafer is dropped directly onto another wafer in the container, for example, both wafers may be scratched, cracked or broken.
Therefore, a need has arisen for an improved method for handling wafers during wafer processing, testing, storing and transporting. A need has also arisen for such a method that reduces the likelihood for scratching, cracking, breaking or otherwise damaging a wafer when the wafer must be handled. A need has further arisen for such a method that reduces the likelihood of vibration, abrasion or other stress being placed on the die of the wafer.